Historically the prosthetic user interface has been a cylindrical socket that merely surrounds the remaining limb part with some contouring of the proximal brim so that it will accommodate the shape of the next proximal joint or body part. Typically this socket is made by taking a plaster cast over the limb and filling it with plaster to form a positive model of the limb. Minor changes are made to this shape to relieve boney prominences. When this model is used to create a socket by laminating or thermoforming a layer of plastic there over, the resulting socket mainly encapsulates the limb part. Conventionally, no modification of the traditional model is done. This opportunity to specifically enhance the resulting structure's ability to impart desired motion to the complete prosthesis, and to prevent undesired motion from occurring, has been overlooked, even though these are the most important functions of the interface. The traditional encapsulating or closed volume socket merely contains the soft tissue but does little or nothing to prevent lost motion between the socket and the underlying skeletal structure.
Some improvements have been made in the traditional interface. In particular, many technicians replace the fully encapsulating outer socket with a frame having one or more openings. This change is accompanied by making the inner socket of a flexible material. The resulting frame-style design usually is more comfortable. New materials such as carbon fiber composites add rigidity where needed especially in open frame designs. New flexible materials allow the socket wall to flex in other areas for comfort. Even when these newer flexible materials are used, the soft liner still fully encapsulates the remaining limb as traditionally done, and thus provides a compressive or elastic force to all of the limb's soft tissue.
Conventional laminations over a plaster model work best when the surfaces of the model are convex facing outward, following the general contours of the outside surface of the limb.